The invention relates to a display screen, especially a flat-panel display screen such as a low-energy cathode ray display screen, which comprises a phosphor composition of a sulphide or selenide-containing phosphor which is provided with a coating.
The trend towards flat-panel display screens has led to low-energy cathode ray display screens which constitute a new development in the field of luminescent-display devices. Flat-panel display screens were developed for three market segments, i.e. office automation, audio/video technology and navigation and entertainment. In the field of office automation reference must be made, in particular, to mobile applications which range from the Notebook Computer, Personal Digital Assistant, fax machine to the mobile telephone. In the field of audio/video, the flat-panel display screens are intended not only for use in camcorders but also in television receivers and monitors. The third field of application comprises flat-panel display screens which are to be used as monitors for navigation systems in cars and aircraft, but also as display screens for electronic games.
In the case of flat-panel display screens, geometric and other aspects have led to a reduction of the maximum accelerating voltage to values which are much lower than in customary cathode ray display screens. As, on the other hand, the picture brightness and energy efficiency of the display screen decreases as the accelerating voltage decreases, said accelerating voltage must not be reduced too much. Low-energy cathode ray display screens are therefore operated at accelerating voltages in the range from approximately 1 to 10 kV. Said accelerating voltages are clearly below those of customary cathode ray display screens, which are operated at a voltage of typically 25-35 kV; however, they are clearly above the accelerating voltage of vacuum fluorescent display screens, which are operated at approximately 0.01 to 0.3 kV.
Phosphors for cathode ray display screens, also when they are of the conventional type, are customarily subjected to a surface treatment so that they can more adequately meet the requirements.
One of the requirements to be met is that the phosphor powders should be readily dispersable in dispersion solutions used in the photolithographic manufacturing processes for display screens. For this purpose, surface coatings are developed which yield free-flowing powders from which stable dispersions can be made in a simple manner.
A further aspect in the development of coatings is the protection against the aggressive chemicals used in the photolithographic manufacturing processes. Particularly in the customarily used "flow coating" process in which dichromates are applied, unprotected phosphors are subject to chemical attack.
Further requirements to be met by phosphors relate to the colour quality. This is the reason why in the so-called "pigmented" phosphors the coating of dispersing aids is replaced by or combined with a second type of coating whose main constituents are colour particles. Said colour particles reduce the reflection of ambient light from the phosphor grains and hence increase the contrast of the image depicted on the display screen. In the case of phosphors whose chromatic values deviate from the standard, said colour particles additionally serve as colour filters which bring the chromatic values in conformity with said standard.
Functionally different coatings are used in the case of phosphors for vacuum fluorescent display screens in which excitation takes place by means of low-energy electrons of several tens of volts to maximally several hundred volts. In this case, charging effects during excitation of the phosphors, which adversely affect the excitation efficiency, must be precluded. Coatings for phosphor powders for vacuum fluorescent display screens are therefore composed of conductivity-improving materials such as indium tin oxide, fine metal powders and such.
It is for example known from JP 3-26781 (A) to provide the surface of fluorescent substances with a complex, electroconductive coating which is preferably composed of 0.1-15 wt. % of complex, electroconductive particles which are manufactured by providing electroconductive, fine particles, which preferably have an average particle diameter of 1.0001-0.1 .mu.m and which comprise one or more elements selected from the group formed by Al, Au, Ag or Cu, on the surface of large electroconductive particles which have an average particle diameter, preferably, of 0.01-1 .mu.m, and which comprise one or more compounds of the group formed by SnO.sub.2, Sb.sub.2 O.sub.3, ZnO, In.sub.2 O.sub.3, TiO.sub.2, Bi.sub.2 O.sub.3, CdS or MoO.sub.3.
However, phosphors used for low-energy cathode ray display screens must meet entirely new requirements. In order to achieve a good picture brightness in spite of the low exciting voltage, these display screens must be operated at high currents. To ensure that the necessary current does not increase to impracticably high values, use must simultaneously be made of phosphors having a high efficiency. Even if the architecture of the display screen permits extremely high current values, the low efficiency of a phosphor cannot be compensated for at will by increasing the current value because, after increasing linearly with the current intensity, the luminous intensity of the phosphor finally attains a saturation value. The current intensity at which this saturation value is attained depends upon the type of phosphor used and on how it must be prepared.
Therefore, it is very important for low-energy cathode ray display screens that the phosphors used exhibit a high efficiency and that they reach said saturation value only at high current intensities. The condition of a high efficiency at a sufficiently high saturation value is fulfilled by only very few phosphors, in particular by sulphide-containing phosphors and selenide-containing phosphors such as ZnS:Ag, ZnS:Cu, ZnCdSe:Ag and such.
However, under the excitation conditions in low-energy cathode ray display screens, said sulphide and/or selenide-containing phosphors are very rapidly degraded, i.e. the bombardment with low-energy electrons leads to an accelerated reduction of the luminous intensity and efficiency of the phosphor. Probably the low penetration depth of the low-energy electrons causes the radiation-induced reactions to be concentrated and intensified in the surface layers of the phosphor grains.
In conventionally coated display screens, which are excited by low-energy cathode rays, a rapid reduction in brightness is observed. In coloured display screens, in which customarily three different phosphors are used, this additionally leads to a slow change of the chromatic values of all mixed colours when each one of the phosphors used does not degrade at an equal rate.
However, commercial low-energy cathode ray display screens, for example color television receivers having a flat display screen should produce images having a constant brightness and pure chromatic values for several thousand hours of operation.